The present invention generally relates to semiconductor devices having level shift diodes, and more particularly to a semiconductor device having at least one level shift diode with an arbitrary level shift quantity by utilizing a discontinuity in the energy band at a heterojunction between two semiconductors.
Conventionally, in a semiconductor device based on a silicon layer, a level shift diode is constructed by utilizing the fact that a forward voltage drop is approximately constant at a pn junction. The level shift diode is used to shift a voltage level by a predetermined value in a logic circuit and the like employing transistors. When a voltage drop introduced by one pn junction is insufficient, a plurality of level shift diodes are connected in series to provide the required voltage drop.
In a semiconductor device based on a gallium arsenide (GaAs) layer, a level shift quantity of a level shift diode utilizing a Schottky junction is a constant value which is approximately in a range of 0.6 V to 0.8 V per Schottky junction. The Schottky barrier height is in the range of 0.6 V to 0.8 V regardless of the kind of metal formed on the GaAs layer. Hence, even when a plurality of Schottky junctions are utilized, it is possible to obtain only a level shift quantity which is an integral multiple of the constant value. When making a Schottky diode, the Schottky junction is made by forming a metal layer on the GaAs substrate. But the surface of the GaAs substrate is exposed to the air when forming the metal layer thereon. The surface of the GaAs substrate includes irregularities when compared to that of the silicon substrate, and for this reason, it is difficult to accurately control the level shift quantity of the Schottky diode because the Schottky diode is easily affected by a leakage current. In addition, since it is necessary to utilize a plurality of Schottky junctions in order to obtain a relatively large level shift quantity, a large area of the semiconductor device is occupied by the Schottky diodes for providing the desired level shift.
Therefore, there is a demand for a semiconductor device which has one or more level shift diodes with arbitrary level shift quantities, and in which the level shift diodes only occupy a small area on the semiconductor device. Needless to say, it is essential that the level shift quantity of each level shift diode on the semiconductor device is accurately controlled to a desired value.
On the other hand, "MONTE CARLO SIMULATION OF GaAs SUBMICRON n.sup.+ -n-n.sup.+ DIODE WITH GaAlAs HETEROJUNCTION CATHODE" by K. Tomizawa, Y. Awano and N. Hashizume, ELECTRONIC LETTERS, Dec. 9, 1982 Vol. 18, No. 25/26 shows a current density versus anode voltage characteristics of a GaAs diode having a Al.sub.x Ga.sub.l-x As heterojunction cathode for different Al contents x.